1. Field of the Invention
The present invention relates to an optical wavelength tunable filter and more particularly to the optical wavelength tunable filter which can be suitably used for an optical wavelength multiplexing transmission system.
2. Description of the Related Art
In recent years, a transmission capacity of one communication line increases as types of information communications are diversified. To meet a need for such increased transmission capacity of the communication line, a variety of technologies have been developed. Among them, a dense wavelength division multiplexing (DWDM) transmission technology capable of transmitting a plurality of lightwave signals by only one optical fiber and/or an optical cross-connect (OXC) technology in particular have received widespread attention.
FIG. 5 is a schematic block diagram showing configurations of a conventional communication system employing the DWDM transmission technology. In this communication system, sending lightwave signals 101 to 10N each having a different wavelength component xcex1 to xcexN input on a sender side are multiplexed by an optical multiplexing section 11. The optical multiplexing section 11 is made up of an array-waveguide grating (AWG) serving as a planar lightwave circuit (PLC). A lightwave signal multiplexed by the optical multiplexing section 11 is amplified by an optical amplifier 12 and is sent out to an optical fiber 13. On a receiver side, the lightwave signal received through the optical fiber 13 is amplified by an optical amplifier 14 and is then input to an optical demultiplexing section 15. The optical demultiplexing section 15 is made up of the AWG as in the optical multiplexing section 11. Generally, by inverting the AWG from its input side to its output side or vice versa, it can be used not only as an optical multiplexer but also as an optical demultiplexer. An amplified lightwave signal input into the optical demultiplexing section 15 is output as received lightwave signals 161 to 16N each having a different wavelength component xcex1 to xcexN. Though the AWG used on the receiver side in the communication system employing the DWDM transmission technology is able to extract light having a specified wavelength component with high accuracy, it is expensive. Moreover, the AWG has a problem in that an easy changing of the wavelength component to be extracted is impossible. To solve this problem, in some cases, an optical wavelength tunable filter capable of extracting light having a specified wavelength from a plurality of rays of light branched by an optical branching device is used as the optical demultiplexer, instead of the AWG. Various types of optical wavelength tunable filters are proposed and one of them is disclosed in Japanese Patent Application Laid-open No. Hei4-168416 under xe2x80x9cAn optical wavelength tunable filterxe2x80x9d.
FIG. 6 is a perspective view showing configurations of the optical wavelength tunable filter employed in the technology disclosed in Japanese Patent Application Laid-open No. Hei4-168416. In the optical wavelength tunable filter disclosed above, an optical waveguide 21 is formed on a substrate 20 having an electro-optic effect. Electrode 22 and electrode 23 are formed by evaporation on both sides of the optical waveguide 21 on an upper face of the substrate 20. The electrode 22 is connected to a terminal of a supply power voltage V. The electrode 23 is grounded. On each of end faces of the optical waveguide 21 are mounted reflecting film 26 and reflecting film 27. The optical wavelength tunable filter having configurations as described above has a Fabry-Perot resonance characteristic upon which a specified wavelength component causing a relative transmittance to be xe2x80x9c1xe2x80x9d is periodically dependent in a state where the reflecting film 26 and reflecting film 27 are mounted at both end face 24 and end face 25 of the optical waveguide 21 and when a voltage is applied between the electrode 22 and electrode 23. Therefore, when a plurality rays of light 28 each having a different wavelength component out of xcex1 to xcexN, branched by the optical branching device mounted in the optical demultiplexing section 15 shown in FIG. 5, are input from the end face 24 of the optical wavelength tunable filter shown in FIG. 6, only lightwave signal having the wavelength component conforming to the specified wavelength component that causes the relative transmittance to be xe2x80x9c1xe2x80x9d is output from the end face 25 as output light 29. Moreover, by changing the voltage V to be applied between the electrode 22 and electrode 23, a refractive index of the optical waveguide is changed by the electro-optic effect. The wavelength component of the output light 29 to be emitted can be changed as well.
Thus, by using the optical wavelength tunable filter described as above for a plurality rays of light each having a different wavelength component out of xcex1 to xcexN, branched by the optical branching device mounted in the optical demultiplexing section 15, only lightwave signal having a desired wavelength component can be extracted at low costs.
However, the conventional optical wavelength tunable filter has a problem in that it requires a substrate having electro-optic effects as a special material, thus making it costly when compared with one fabricated using a silica based material with properties being equivalent to optical fiber. Moreover, another problem is that the conventional optical wavelength tunable filter can be made less integrated when compared with one fabricated using the silica based material.
Thus, a demand is increasing for the optical wavelength tunable filter which can be formed on a substrate made of the silica based material allowing low-cost production and high integration.
In view of the above, it is an object of the present invention to provide an optical wavelength tunable filter which can be highly integrated and fabricated at low costs.
According to a first aspect of the present invention, there is provided an optical wavelength tunable filter including:
an optical waveguide having a first branched optical waveguide and a second branched optical waveguide merging into one piece of the optical waveguide through which optical wavelength multiplexed signals each having a different wavelength component being incident from an end face of the first branched optical waveguide are propagated;
one comb-type electrode or a plurality of comb-type electrodes each corresponding to each of the different wavelength components, mounted vertically to a propagating direction of the optical wavelength multiplexed signals leaving a specified space apart from the optical waveguide formed by the merger of the first branched optical waveguide with the second branched optical waveguide; and
a voltage applying circuit to apply a predetermined voltage to each of the comb-type electrodes.
By configuring as above, since a refractive index of the optical waveguide is changed by an electric field produced by the voltage applied to the comb-type electrode and since only light having a diffraction wavelength contained in the optical wavelength multiplexed light can be reflected, the light having any wavelength component can be extracted by simplified configurations of the optical wavelength tunable filter as provided by the present invention.
According to a second aspect of the present invention, there is provided an optical wavelength tunable filter including:
an optical waveguide formed in parallel to an upper face of a substrate made-of a silica based material having a first branched optical waveguide and second optical branched waveguide merging into one, through which optical wavelength multiplexed signals each having a different wavelength component being incident from the first branched optical waveguide are propagated;
one comb-type electrode or a plurality of comb-type electrodes each corresponding to each of the different wavelength components, mounted on the substrate vertically to a propagating direction of the optical wavelength multiplexed signals leaving a specified space apart from the optical waveguide formed by the merger of the first branched optical waveguide with the second branched optical waveguide;
an antireflection terminal mounted facing an end face of the optical waveguide formed by the merger of the first branched optical waveguide with the second branched optical waveguide; and
a voltage applying circuit to apply a predetermined voltage to each of the comb-type electrodes.
By configuring as above, since the silica based material with properties being equivalent to an optical fiber is used, the optical wavelength tunable filter can be provided which is excellent in matching in terms of properties of materials used and which allows low-cost extraction of light having any wavelength component and easy integration. Moreover, lightwave signals having an unwanted wavelength component can be terminated with no reflection.
According to a third aspect of the present invention, there is provided an optical wavelength tunable filter including:
one first optical filter or a plurality of first optical filters each having an optical waveguide formed in parallel to an upper face of a substrate made of a silica based material having a first branched optical waveguide and a second branched optical waveguide merging into one, through which optical wavelength multiplexed signals each having a different wavelength component being incident from the first branched optical waveguide are propagated and one comb-type electrode or a plurality of comb-type electrodes each corresponding to each of the different wavelength components, mounted on the substrate vertically to a propagating direction of the optical wavelength multiplexed signals leaving a specified space apart from the optical waveguide formed by the merger of the first branched optical waveguide with the second branched optical waveguide, wherein light emitted from the optical waveguide formed by the merger of the first branched optical waveguide with the second branched optical waveguide disposed in a preceding stage is incident into the first branched optical waveguide disposed in a subsequent stage; and
a second optical filter having an optical waveguide through which optical wavelength multiplexed signals each having a different wavelength component being incident from a third branched optical waveguide, which is merged with a fourth branched optical waveguide into one optical waveguide, to which light emitted from an end face of the optical waveguide formed in parallel to the upper face of the substrate by the merger of the first branched optical waveguide with the second branched optical waveguide disposed in a final stage of the first optical filter is incident, are propagated, an antireflection terminal mounted facing an end face of the optical waveguide formed by the merger of the third branched optical waveguide with the fourth branched optical waveguide, one comb-type electrode or a plurality of comb-type electrodes each corresponding to each of the different wavelength components, mounted on the substrate vertically to a propagating direction of the optical wavelength multiplexed signals leaving a specified space apart from the optical waveguide formed by the merger of the third optical waveguide with the fourth optical waveguide and a voltage applying circuit to apply a predetermined voltage to each of the comb-type electrodes.
By configuring as above, a plurality of lightwave signals each having the specified wavelength component can be extracted simultaneously from the first branched optical waveguide of the first filter and from the fourth branched optical waveguide of the second optical filter, from the same multiplexed input light.
In the foregoing, a preferable mode is one wherein a feedback light intercepting means is mounted to the end face of the branched optical waveguide, out of the first branched optical waveguide and second branched optical waveguide, into which the optical wavelength multiplexed signals each having a different wavelength component are input.
By configuring as above, degradation of a signal to noise ratio can be avoided.
Also, a preferable mode is one wherein at least one factor out of a width of each comb-type electrode, an amount of clearance between comb-type electrodes and number of comb-type electrodes is determined based on diffraction wavelength and wherein refractive index of the optical waveguide is changed, in a manner as if changed by a diffraction grating, by an electric field generated by the voltage applied by the voltage applying circuit used to apply the voltage corresponding to each wavelength component.
By configuring as above, the lightwave signals having the corresponding diffraction wavelength can be obtained by being reflected by the electrode to which the predetermined voltage is applied by the voltage applying circuit. Moreover, both low costs in production of the filter and highly precise light extraction can be achieved at a same time.